On 12/7/2024 06:31, Paul George wrote:
In the example below, I noticed that the JOIN predicate "t1.a<1" is not
pushed down to the scan over "t2", though it superficially seems like it
should be.
It has already discussed at least couple of years ago, see [1].
Summarising, it is more complicated when equivalences and wastes CPU
cycles more probably than helps.
create table t as (select 1 a);
analyze t;
explain (costs off) select * from t t1 join t t2 on t1.a=t2.a and t1.a<1;
QUERY PLAN
-------------------------------
Hash Join
Hash Cond: (t2.a = t1.a)
-> Seq Scan on t t2
-> Hash
-> Seq Scan on t t1
Filter: (a < 1)
(6 rows)
The same is true for the predicate "t1.a in (0, 1)". For comparison, the
predicate "t1.a=1" does get pushed down to both scans.
explain (costs off) select * from t t1 join t t2 on t1.a=t2.a and t1.a=1;
QUERY PLAN
-------------------------
Nested Loop
-> Seq Scan on t t1
Filter: (a = 1)
-> Seq Scan on t t2
Filter: (a = 1)
(5 rows)
[1] Condition pushdown: why (=) is pushed down into join, but BETWEEN or
>= is not?
https://www.postgresql.org/message-id/flat/CAFQUnFhqkWuPCwQ1NmHYrisHJhYx4DoJak-dV%2BFcjyY6scooYA%40mail.gmail.com
--
regards, Andrei Lepikhov
